Wood pulp

As is known, the resulting pulp in the chemical pulping of fibrous plant fiber material consisting mainly of cellulose.

Pulp is, in addition to pulp, a key raw material in papermaking. 90 % of the pulp produced worldwide is made from wood. The use of annual plants such as straw, bagasse, kenaf and bamboo as raw material goes worldwide significantly.

The production of pulp is carried out regionally differently from industrial waste wood or plantation timber. In Europe and North America saw mill waste (mostly softwood ) used, for example. In South America, South Africa and Australia Hardwood is often used in forest plantations ( fast-growing eucalyptus). The wood is first processed to shredded wood ( wood chips) and then chemically digested. is predominant

• The alkaline sulfate process, are manufactured with the over 95 % of world production. Both deciduous and coniferous wood can be used. There are also
• The acidic sulfite process. Only 2 % to 3% are produced by this process. The digestion of highly resinous wood such as pine wood, this is rare. Most spruce, beech or eucalyptus wood is used. The sulfite process has a stable market niche in the production of viscose pulp.

A distinction is made between quality softwood and hardwood pulp. Softwood kraft pulp has long, strong fibers of an average of 3.3 mm in length and are thus a paper reinforcement. Hardwood pulp has shorter fibers of about 1 mm in length. It is suitable for the manufacture of smooth papers ( writing paper, copy paper ) or tissue ( tissue paper ).

History

Paper was initially made ​​exclusively from rags. In the 19th century, the increasing demand for paper led to the development of wood pulp and the pulp.

The digestion of wood with caustic soda ( NaOH) was developed in 1851 by H. Burgess and C. Watts in England and introduced in 1854 in the United States. The high caustic soda demand made ​​the evaporation, burning and re caustification too expensive. C. Dahl in Danzig tried to insert directly into the recovery system sodium sulfate ( Na2SO4). The resulting reduction by sodium sulfite ( Na2SO3 ) improved fiber quality significantly. 1884 Dahl received a patent for the process modification. The alkaline conditions led to very strong fibers, however, were dark brown in color. They were therefore used long only for the production of unbleached paper products ( paper bags, cardboard boxes ).

1867 sulfite pulping for BC Tilghman was patented in the U.S. ( USP 70.485 ). This method uses a solution of calcium bisulfite (Ca ( HSO 3 ) 2) and free sulfur dioxide ( SO2). The first industrial-scale plant was built in 1874 in Sweden under CD Ekman, she took small, rotating cooker and magnesium bisulfite (Mg ( HSO 3 ) 2). In Germany, the sulphite process by Tilghman Mitscherlich was introduced in 1875. Mitscherlich used lying cooker, low temperatures of 110 ° C and very long rotation periods. The acidic sulphite according to Mitscherlich leads to comparatively bright fibers. When using wood chips from debarked wood good a whiteness of almost 70 % ISO is achievable. For further elucidation of the fibers, the chlorine bleach with bleaching powder was in use.

Production

For the production of pulp as evenly shaped chips are needed. Mechanical comminution improves uniform impregnation of the wood with the digestion solution. Very coarse chips are not completely impregnated, resulting undigested debris. For fine wood chips have the disadvantage of a strong mechanical shortening of the fibers. Sawmill waste ( splinters ) or chipped wood are hardly suitable. Wood consists mainly of lignocellulose. In the digestion of the lignin the incrustations are broken and released, while the structure of the cellulose is largely retained in the cellulose matrix.

Sulphate process

The sulphate process (including digestion or sulfate process ) is also called kraft process due to the stronger fibers. It is named after the " make-up chemical " of the process, the Glauber's salt (sodium sulfate, Na2SO4 ), named. The active substances are sodium hydroxide and sodium sulfide. The digestion is carried out in continuous digesters mainly. Also, modifications of the original batch process to be applied. The wood chips are impregnated with the cooking liquor and fed from above the reactors to. At a temperature up to 170 ° C the wood components, such as hemicelluloses and lignins, alkaline solved. The lignin is depolymerized by nucleophilic ether cleavage of the sulfite ion ( SO32 - ). A degradation of the cellulose fibers is undesirable, so the digestion process is terminated before the entire lignin is dissolved. In unbleached pulp therefore about 2% are listed to 3 % lignin. In a production line, a plant for pulp production up to 1.3 million tons of pulp are produced annually.

Sulfite

The sulfite to ( or even sulfite pulping process ) required sulfur dioxide was formerly produced by direct combustion of sulfur or roasting of sulphide ore and passed through a wooden tower with limestone. By sprinkling with cold water, the required Calciumbisulfitlösung with additional free sulfur dioxide formed. Today, liquid sulfur dioxide is used. The acid sulphite dissolves the bond between lignin and cellulose by sulfonation and ether cleavage of the lignin. The low pH damages the structure of the cellulose chains at the acetal between the sugar molecules. The crystalline region of the cellulose remains stable. The fiber strength of Sulfitzellstoffes is worse than in sulphate pulp. The hydrophilic sulfonated lignin is water soluble and can be further processed to lignosulfonates. These come with disperse dyes as aids for use.

For the preparation of a paper pulp bleaching is almost always necessary to dissolve the lignin residues by oxidation. The sulfite are dissolved by the alkaline bleaching additionally hemicelluloses.

The yields per dry mass of wood be in the sulfate process 40 % to 45 % and with sulfite 45% to 50 %. The wastewater pollution is directly correlated with the amount of dissolved substance.

Other methods

Other methods, such as soda (Na2CO3 · 10H2O ), and the soda / anthraquinone process (soda / AQ process ), where the anthraquinone as a catalyst for a better delignification is technically irrelevant. Also, the alkaline sulfite pulping with addition of anthraquinone and methanol ( ASAM process ) or Natural pulping process, which operates with performic acid and the like Acetocell method with a peroxyacetic acid - decomposition may be mentioned. These methods have been tested in pilot plants, but are not economical.

Also the organocell method with sodium hydroxide and methanol ( CH4O ) proved to be not competitive. A full-scale plant based on this principle in Kelheim was put into operation with substantial government funding in 1992, but shut down in 1993 due to quality problems.

An eco-friendly approach represents the tentatively performed digestion using formic and acetic acid

Chemical recovery

In the classical Mitscherlich process, the waste liquor of calcium sulfite pulping has been disposed of in the receiving water. Thus, half of the mass of wood used contaminated waste water and water bodies. Modern pulp mills recover the pulping chemicals. Only the acidic magnesium sulfite process is carried out - Due to the formation of calcium sulfate deposits ( gypsum) is in Central Europe - with a few exceptions. After the " digestion " of the pulp is washed and concentrated by evaporation, the waste water. This gives the acidic " black liquor ". In the " combustion " magnesium is cleaved thermally in magnesium oxide ( MgO) and sulfur dioxide. An electrostatic filters MgO is converted and deposited in reactors with the venturi in the flue gas SO2 back to magnesium bisulfite (Mg ( HSO 3 ) 2).

During combustion, the " black liquor " (black does not refer to the color, but the low purity ) under reduced air entry creates a melt of sodium carbonate ( Na2CO3) and sodium sulfide ( Na2S ). By dissolving in water gives " green liquor ". The sodium carbonate is causticized with calcium hydroxide ( slaked lime, Ca (OH) 2) sodium hydroxide solution. By filtration creates the " white liquor ". It is used again in the process. The filtered calcium carbonate is calcined in a rotary kiln back to calcium oxide ( quicklime, CaO ).

This recovery not only relieves the receiving water, but from the waste liquor, the process energy required is obtained. Modern plants produce a surplus of electrical energy. The excess steam can be used in an integrated pulp and paper production for paper drying.

Small pulp mills usually have no recovery plant. The strong environmental pollution caused by the high content of lignin and hemicelluloses in the wastewater is increasingly leading to decommissioning. In China alone, were closed by the government in the past ten years, over 10,000 straw pulp mills with an annual production of less than 5,000 tons due to the pollution of rivers and lakes. The cost for the recovery of chemicals is too high for small plants.

Pulp quality

The strength of the pulp fiber is suitable for making some types of paper crucial. Cellulose in order to improve the paper strength is referred to as Armierungszellstoff. He has long and thin fibers. Softwood from regions with cool winters, but warm summer has these properties. This pulp (magazine, catalog paper) used in the manufacture of wood-containing paper. In regions with a long growing season result softwood fibers, long and both are relatively thick. These coarse fibers are ideally suited for producing solid paper bags and coffee filters.

The short fibers of hardwoods provide a smooth, even surface of the paper. On paper machine quality paper can be made ​​entirely of hardwood pulp. For older machines mixtures of long and short fibers are needed.

The pulp production process has a significant influence on the fiber quality. Quality loss caused by fiber shortening of the wood chips to intensive digestion or in pulp bleaching. Quality values ​​of mechanical strength tests are breaking length or tear strength. In addition, the relationship between the degree of polymerization of cellulose and the fiber strength. The analysis of the viscosity of a fiber sample dissolved are indications of the degree of fiber damage.

Trade in pulp

Pulp is traded predominantly bleached. In 2004, world production was estimated at 80 million tons of bleached. Leading today ECF ( Elemental Chlorine Free) bleached grades with a brightness of 88 % to 90 % ISO for softwood pulp and 90 % ISO for hardwood pulp. For the production of photo paper high white hardwood pulps are used with a brightness of 92 % ISO.

Another quality feature is the stability against yellowing. This can be with TCF (total chlorine free ) bleached hardwood pulp low.

Use

• Paper
• Carton
• Filter paper ( mostly in the household filter bags )
• Cotton wool and bandages
• Tissues
• Hygiene products such as tampons, sanitary napkins, diapers and aids in urinary or fecal incontinence
• Celluloseregnerat fibers such as viscose, lyocell, modal, rayon or cuprammonium rayon
• Reinforcement of gypsum or cement as a composite material
• Cellulose derivatives such as cellulose ester or ether,
• Pulp composite element
• Chemical pulp as raw material for cellulose deformation and derivatization, and for producing Celluloseregeneraten (for example viscose, films)